Dye-sensitized solar cells utilizing a sensitizer dye have attracted wide attention. Dye-sensitized solar cells include, for example, a transparent conductive film, a porous semiconductor electrode having a sensitizing dye supported therein, a hole transport layer, and a counter electrode formed in this order on a transparent substrate.
An example of such cells is described in literature: O'Regan, B. and Grätzel, M. (1991) Nature, 353, 737. The solar cell contains, in this case, a pair of opposite electrodes (an anode and a cathode) and an electrolyte in between them. The electrolyte includes iodide ion couples having different oxidation states as a mediator of holes generated upon charge separation at the dye-sensitized nanoporous semiconductor-electrolyte interface. The cathode is made of a conductive material whilst the anode is made of a plate of transparent materials such as glass having on its surface a transparent conductive layer of light-transmitting tin dioxide (SnO2) which might be doped with another element and also a semiconducting titanium dioxide (TiO2) layer thereon. The TiO2 layer is formed with a TiO2 semiconductor consisting of nanocrystalline particles, to the surface of which sensitizing dyes are attached. When the interface between the TiO2 nanocrystalline layer and dyes is irradiated, electrons are injected to TiO2. On the other hand, the mediator undergoes oxidation within the electrolyte; three iodide ions (I−) eject two electrons, resulting in triiodide ions (I3−) of high oxidation degree. The electrons are then transported through the TiO2 nanocrystalline layer and collected by the transparent conductive layer whilst the triiodide ions (I3−) diffuse to the cathode, and obtain two electrons to be reduced into the iodide ions (I−). Thus, this type of wet cell converts solar energy into electric energy.
Dye-sensitized solar cells have been expected to serve as a solar cell for the next generation because of simplicity and convenience of fabrication methods thereof, reduced material costs therefore and the like.
In order for dye-sensitized solar cells to be put in practical use, there has been a demand for further improvement in conversion efficiency, and for that, there has been a demand for an increase in the current to be generated (short-circuit current, Jsc), in open-circuit voltage (Voc) as well as in safety and durability.
The dye-sensitized solar cells that perform best presently contain at least one volatile organic solvent to reduce the viscosity of the electrolyte, thus to enhance the ion mobility. The greatest challenge here is to remove or reduce the volatility of electrolytes by replacing the volatile solvent with ionic liquids with the goal that the electrolyte liquid composes only of ions such as disclosed e.g. in Yu Bai et al., “High-performance dye-sensitized solar cells based on solvent-free electrolytes produced from eutectic melts”, Nature Materials 2008, 1.
One of the factors that limits the power conversion efficiency of ionic liquid-based DSSC may be the much larger extent of recombination of the injected electron in the semiconductor or conduction band electrons (e.g. TiO2) due to the much larger amount of the oxidized part of the redox couple (e.g. I3−) at the relevant operating condition. In order to increase the open-circuit voltage, it is necessary to avoid such recombination which means in other words to suppress the leakage current at the semiconductor electrolyte junction.
The leakage current arises from the described reduction of triiodide by conduction band electrons (e−cb):I3−+2e−cb(TiO2)→3I−which occurs despite the fact that the TiO2 surface is covered by a monolayer of the dye. The triiodide, due to its relatively small size, either crosses the dye layer or has access to nanometersized pores onto which the dye cannot completely cover, i.e. where the surface of TiO2 is bare and exposed to redox electrolyte.
Typically, recombination is suppressed by addition of compounds which are considered to be coordinated to free sites on the TiO2 surface, thus blocking the access of triiodide or free iodine to potential recombination sites. Examples of compounds typically used for this purpose are (poly)ether derivatives, amides, esters, nitriles. The currently best results have been achieved using N-alkyl-benzimidazoles (Zhaofu Fei et al., Inorg. Chem. 2006, 45, 10407-10409, A Supercooled Imidazolium Iodide Ionic Liquid as a Low-Viscosity Electrolyte for Dye-Sensitized Solar Cells), 4-tert-butylpyridine (TBP, M. K. Nazeeruddin et al., J. Am. Chem. Soc. 1993, 115, 6382-6390) or nitrogen containing heterocyclic additives such as tetrazole, pyrazole, triazole, pyrazine, pyrimidine, triazine (H. Kusama et al., J. Photochem. Photobiol., A Chemistry, 2005, 169-176). TBP and the nitrogen containing heterocyclic additives are described as useful especially in electrolytes comprising at least one volatile organic solvent.
It is believed that the donating properties of the nitrogen lone pair in the heterocyclic additives known so far are responsible for the enhanced Voc.
US 2005/0150544 describes a dye-sensitized solar cell wherein the used dye is Ruthenium 535-bisTBA dye, made by Solaronix Col, Swiss and the electrolytic solution based on acetonitrile contains a heterocyclic compound containing an oxygen atom in the ring, such as tetrahydrofuran, 2-methyl-tetrahydrofuran, pyran, tetrahydropyran, furan, 2-methyl-furan, 1,4-dioxane, trioxane, 4-methyl-1,3-dioxolane, 1,3-dioxolane, 1,3-dioxane, 2H-1,3-dioxole, 3H-1,2-dioxole, dioxene, 1,4-dioxin, trioxane, and the concentration of the heterocyclic compound in the electrolytic solution is 5 to 40% by volume.
When TBP is used, a poisoning against the counter electrode is observed. In addition, TBP is volatile which means it influences the stability and durability of the device.
When other heterocyclic additives are used, a marked decrease in short-circuit current is observed, and the actually obtained open-circuit voltage is low.
However, there continues to be a demand for new and/or improved additives which are able to improve open circuit voltage by shifting the conduction bandedge of oxide semiconductor in a negative direction and/or by reducing the leakage current and therefore maximising the maximum power-operating voltage. The open circuit voltage is one important parameter which needs to be tuned in achieving improved DSC efficiency over a broad temperature range including temperatures above room temperature and well below the temperature at which dye desorption may take place (i.e. in the range of 40° C. to 120° C.)).
JP 2006/202646 mentions 4-phenyl-pyridine-N-oxide as possible additive in electrolytes for various devices for instance lithium secondary batteries, electrolytic condenser, electric double layer capacitor etc. It is mentioned that electrolytes containing the additives according to JP 2006/202646 including accidentally 4-phenyl-pyridine-N-oxide can demonstrate heat resistance and no colour change while heating. There is no hint that tertiary N-oxides according to formula I may advantageously be used in dye-sensitized solar cells and that they have the capability to increase the Voc.